O,O-diethyl-O-(2-isopropyl-4-methyl-pyrimid-6-yl)-thionophosphoric acid ester [Diazinon] is a known compound described, for example, in U.S. Pat. No. 2,754,243 and in Zeitschrift fur Naturforschung 1953, 8b, 255-232.
Since many insects develop resistance to insecticides as, for example, to phosphoric acid esters, investigations have centered on developing compounds and processes to overcome this resistance. In the case of Diazinon, for example, compounds which would synergize the activity of Diazinon were sought. The pyrethroids are one class of compounds found to act as synergists for Diazinon. Examples of such compounds useful as synergists include:
3'-phenoxy-.alpha.'-cyanobenzyl .alpha.-isopropyl-4-chlorophenylacetate [Fenvalerat], a compound which is disclosed in DOS No. 2,335,347 and PA0 3'-phenoxy-.alpha.'-ethynylbenzyl .alpha.-isopropyl-4-chlorophenylacetate, a compound which is disclosed in DOS No. 2,365,555.